Solid state voltage control system for electrostatic precipitators

ABSTRACT

A system is provided for automatically controlling power delivery to an electrostatic precipitator. This system includes an SCR power controller which drives a transformer-rectifier set. The output of the transformer-rectifier set is connected to the precipitator. A current transformer monitors the current flow in the transformer and when a certain current level is exceeded, a solid state current limiter responds to limit operation of the gate circuit until a proper level is achieved. Circuitry is also provided for detecting the occurrence of sparking in the precipitator and generating pulse signals as a result thereof. The pulse signals are fed to a solid state logic circuit that quickly shuts off the SCR gate circuit, then subsequently causes soft start to full power.

United States Patent [191 Muskovac Apr. 15, 1975 [75] Inventor: Nicholas G. Muskovac, Rockville.

[73] Assignee: Vectrol Inc., Rockville, Md.

[22] Filed: Aug. 14, 1973 [21] Appl. No.: 388,084

A [56] References Cited UNITED STATES PATENTS l/1968 Cavanaugh 323/24 UX 4/1970 Hall et al. 55/105 5/1971 Drenning et al. 55/105 POWER LINES 3,772,853 11/1973 Burge et a1. 55/105 Primary ExaminerA. D. Pellinen Attorney, Agent, or FirmMorris Liss [57] ABSTRACT A system is provided for automatically controlling power delivery to an electrostatic precipitator. This system includes an SCR power controller which drives a transformer-rectifier set. The output of the transformer-rectifier set is connected to the precipitator. A current transformer monitors the current flow in the transformer and when a certain current level is exceeded, a solid state current limiter responds to limit operation of the gate circuit until a proper level is achieved. Circuitry is also provided for detecting the occurrence of sparking in the precipitator and gener' ating pulse signals as a result thereof. The pulse signals are fed to a solid state logic circuit that quickly shuts off the SCR gate circuit, then subsequently causes soft start to full power.

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SOLID STATE VOLTAGE CONTROL SYSTEM FOR ELECTROSTATIC PRECIPITATORS FIELD OF THE INVENTION The following invention relates to voltage control systems, and more particularly to a solid state voltage control system for powering electrostatic precipitators.

BRIEF DESCRIPTION OF THE PRIOR ART In many industrial processes, gas or smoke is produced having particulate matter that must be removed to achieve air pollution abatement. Electrostatic precipitators have been widely used in this capacity. Precipitators may be mounted in smokestacks or other gas and smoke conduits. By way of example. one manufacturer of these electrostatic precipitators is the Koppers Corporation. The main scientific principle behind these precipitators is the generation of high voltages on metallic precipitator plates. Adjacent plates are oppositely charged, electrostatically, to attract particulate matter from a smoke or gas stream. Occassionally, the plates are mechanically vibrated so that the accumulated particulate matter is shaken from the plates and collected. A primary problem in the use of electrostatic precipitators relates to power surges in the voltage control system as particulate matter accumulates on the adjacent precipitator plates resulting in arcing. As will be appreciated, the system must respond very quickly to this arcing situation to prevent power surges that might destroy the system. Other factors that are important to consider relate to achieving high component reliability and system stability over a wide range of operating voltages.

U.S. Pat. No. 3,507,096 relates to an improvement in the prior art, whereby this patent recognizes the desirability of utilizing SCR components to provide controlled power to a transformer-rectifier set that energizes the electrostatic precipitators. This patent further employs a magnetic device in the form of a reactor to limit current in the circuitry so that the circuitry is prtected from burn out.

Although the patented subject matter is designed to operate satisfactorily, there are several disadvantages to the use of the reactor. Initially, greater cost is a factor when comparing utilization of the magnetic device as opposed to solid state non-magnetic circuits. An additional problem is the response time of the circuitry. Response for these devices is somewhat slower than for solid state devices, due to the hysteresis effect. Otherwise stated, rapid response by the reactor is slowed due to magnetic reluctance.

BRIEF DESCRIPTION OF THE PRESENT INVENTION The present invention is an improvement of voltage control systems for electrostatic precipitators, as described in the previously mentioned US. Pat. No.

3,507,096. The present invention does not employ a magnetic device, in the form of a reactor to limit current in the system. Rather, solid state switching devices are used. These obviate the disadvantages of the prior art. As a result, a system having maximum performance is made available.

BRIEF DESCRIPTION OF THE FIGURES FIG. 1 is an electrical schematic diagram illustrating the total system for voltage control of electrostatic precipitators. Several of the control circuits are indicated in blocks.

FIG. 2 is a plot of a signal appearing at the output of the system when a spark occurs.

FIG. 3 is a plot of the signal at the input to a transformer-rectifier set, the plot particularly demonstrating a soft start condition.

FIG. 4 is a block diagram of the control portion of the system.

FIG. 5 is a plot of a pulse train generated from the logic circuitry of FIG. 4, the pulses of the train being initiated when a spark at the precipitator is developed.

FIG. 6 is an electrical schematic diagram of a soft start circuit, as shown in FIG. 4.

FIG. 7 is a logic diagram ofa logic circuit which controls the soft start circuitry.

FIG. 8 is an electrical schematic diagram of a current limiting circuit which is responsive to current flow in the primary of a transformer-rectifier set.

FIG. 9 is a plot showing the phase relationship between the output of a gate drive and the AC line signal existing in the system.

FIG. 10 shows alternate circuitry for spark detection.

DETAILED DESCRIPTION OF THE INVENTION Referring to the figures and more particularly FIG. 1 thereof, reference numerals I0 and 12 denote adjacent electrostatic plates that become electrostatically charged with opposite polarities to attract particulate matter from smoke or gas I4 that flows between the precipitator plates. Usually, the precipitator plates are positioned in adjacent parallel relationship and in a typical installation, many such pairs of parallel plates exist. Occassionally, the plates are vibrated or otherwise mechanically moved to shake the accumulated particulate matter therefrom. Then, this particulate matter is col- Iected for proper disposal. It is to be clearly understood that the mechanical portion of the system, namely the precipitator plate assembly and vibrating mechanisms do not form a part of the present invention. This type of equipment is available from several manufacturers, including the Koppers Corporation. The present invention is more particularly directed to the voltage control system for the electrostatic plates.

Power leads l6 and 18 provide electrical energization for the system shown in FIG. 1. Typically, 440 volts are supplied to such a system. A main power control unit 20 furnishes controlled power to the primary 22 of a transformer 24. The secondary of the transformer 26 is fed to a rectifier, such as the diode rectifier 28. The rectifier performs a full wave rectification of the transformed line signal. The transformer 24 and the rectifier 28 are referred to as a transformer-rectifier set. The output from the rectifier is shown at lines 30 and 52. These lines are directly connected to the plates of the precipitator to cause electrostatic charging of the plates, with opposite polarities.

An inductor 34 is added in series with the primary 22 of transformer 24. This inductor adds impedance to the primary circuit so that rapid changes in current to a value exceeding a predetermined threshhold is restrained until the SCR gate 36 can become operative.

Aresistor 38 is connected between point 44 in the rectifier 28 and ground. A voltage develops across the resistor in accordance with the current flowing through the rectifier. This voltage is an indication of electrical parameters existing at the precipitator plates. For example, in the event particulate matter builds up across the plates to effect a very narrow gap, sparking may occur. A typical resulting signal across the resistor is shown and indicated by reference numeral 48. This signal represents a voltage pulse between points 44 and ground 42, the two points defining the ends of resistor 38. Leads from these points are connected to logic circuitry generally indicated by 46, which detect the occurrence of this pulse, and upon such detection, the system is prepared to limit the operation of SCR gate 36 until there is no further arcing.

FIG. 2 illustrates the signal at point 30. The plot shown in FIG. 2 illustrates the occurrence of a pulse signal 48 as just described. When the threshhold is exceeded. the logic circuitry 46 (FIG. 1) ensures a momentary disabling of the SCR gate 36, after which time there is a soft start condition. This means that the energization to the SCR gate 36 builds up gradually as shown. FIGS. 2 & 3 show the time relationship in a soft start condition. between the signals at points 30 and point 22.

FIG. 4 shows, in greater detail, the circuitry of the control unit (FIG. 1). As indicated, power lines I and 2 drive the unit. In FIG. 4, these lines are referred to by 16' and 18'. These lines have respective circuit breakers 54 and 52 serially connected therein. After the breakers are switched on, the lines are energized and supply power to the control unit 20. The power lines are connected to a power supply 56 that converts the 440 volts AC to lower AC and DC levels. One of the AC tap off points from supply 56 is connected to the soft start circuit 58. The purpose of this circuit is to provide slowly increasing power levels to the gate drive 60 after the breakers 52 and 54 have been closed.

A conventional current transformer 64 is positioned over line section 63, in series with the primary 22 of transformer 24. Through inductive coupling, excessive current flow through the primary will cause a response through the current transformer 64 along line 66. This line provides an input to the current limiter 62. The current limiter 62 responds to this excessive current condition by bleeding current from the soft start circuit 58. As a result, the gate drive 60 will not be energized, or will be greatly reduced, therefore preventing, or reducing to a safe operating level, power delivery to the SC R gate 36.

Spark rate control logic 68 is provided to detect the occurrence of the spike type pulses 48 which occur at point 44 (FIG. 1) when sparking occurs at the precipitator plates. The input to the logic 68 is along lead 67, the-opposite end of this lead being connected to point 44 (FIG. 1).

Referring to FIG. 5, there are illustrated a series of spark pulses 48, which provide an input along lead 67 to the logic 68. The output of the logic is a pulse train 72, also shown in FIG. 4, each pulse having constant width and amplitude. However, the repetition of the pulses in the square wave train depend upon the repetition rate of the spark pulses. An operational amplifier 74 has a first input connected to the output of logic 68 so that the operational amplifier receives the pulse train. A second input to the operational amplifier is connected to a reference or spark rate adjustment. The purpose of the operational amplifier is to provide an integration or averaging of the pulse train pulses. This average is a DC level 70 indicated in FIG. 5. Thus, the average voltage is the signal that is present at the output 76 of the operational amplifier 74.

FIG. 6 illustrates in schematic form, the soft start circuit generally indicated by reference numeral 58 in FIG. 4. Input leads 78 are connected to the power supply 56 as shown in FIG. 4. A transformer 80 transforms the input voltage to the secondary 82 of the transformer. The resulting voltage is rectified by diode 84 and the signal is filtered by the parallel RC combination 86, 88. The capacitor 88 charges almost immediately after power is applied to the soft start circuit. The resistor 90 and shunt connected capacitor 92 provide a time constant to slowly charge the capacitor 92. This charging process occurs each time there is an interruption of power from the power supply 56 and thus the power lines 18 and 16 in FIG. 4. Such interruptions occur when the system is first turned on, or in the case of emergency disruptions. The base of buffer transistor 96 is indicated by reference numeral 100 and is connected to the junction between resistor 90 and capacitor 92. Point 102 on the opposite end of capacitor 92 as well as point 100 are connected to the logic circuit 68, as will be explained hereinafter. However, point 102 is connected as a first input to the gate drive 60, while the emitter of buffer transistor 96 furnishes a second input to the gate drive 60. The collector of transistor 96 receives the DC level output from the amplifier 74. Typical output signals from the gate drive are shown at output leads 138 and 140 which are connected to the SCR gate 36 (FIG. 4). The signals on the leads 138 and 140 are trigger signals for the SCR gate 36 and their phase relation to the AC line signal determines the power output from the SCR gate 36. The soft start circuit shown in FIG. 6 becomes reset when the capacitor 92 discharges rapidly through the shunt connected diode 94.

FIG. 7 illustrates the circuitry involved in the logic circuit 68. Upon the occurrence of the spark pulse 48 at input lead 67, a connected one shot 106 generates a squarewave pulse 112, which, by way of example,

may be one-tenth second in duration. The output lead 69 from one shot 106 drives the base of the transistor 110. The collector of the transistor is indicated by reference numeral 100. The emitter of the transistor is indicated by 102. Points 100 and 102 are connected to similarly indicated points in FIG. 6. Thus, the logic 68 is connected with the soft start circuit to achieve a soft start power buildup in the event spark pulses occur, indicating sparking at the precipitator plates. The one shot 106 serves to detect quench time or off time of the individual SCRs 142 and 144 in the SCR gate generally.

indicated by 36 in FIG. 4. The one shot 108 has its input connected with the input of one shot 106. The one shot 108 generates a pulse train 72 for the purposes of spark rate detection and control. The pulse train is fed to the operational amplifier 74 which has already been explained as achieving an integrating or averaging of the pulse train. Referring back to transistor 110, it should be noted that signals from the output leads 100 and 102 that are connected to similarly indicated points in FIG. 6, turn off transistor 96 and keep the transistor turned off for the duration of pulse 112.

FIG. 8 illustrates the circuitry utilized in the current limiter 62 (FIG. 4). As previously mentioned, a conventional current transformer 64 is slipped over a line section 63 of the transformer primary 22 (FIG. 1). The leads from this current transformer 64 are connected to the primary 116 of a high step-up transformer 114.

The secondary of the transformer 118 and a parallel connected resistor 119 are connected to a diode rectifier 120 for performing full wave rectification of the stepped-up voltage signal across the secondary 118. The rectifier 120 has its output points connected across a current limit adjust potentiometer 122. The output from the potentiometer 122 is connected to a resistor 134 that has a shunt capacitor 124 connected thereto to form an RC combination for filtering the signal derived from the full wave rectifier. A Zener diode 126 is connected between the node of the RC combination and the base of the transistor 128. The Zener establishes a threshhold reference. When the signal presented to the Zener exceed the threshhold. transistor 128 is caused to conduct at collector lead 130 and emitter lead 132. By adjusting the potentiometer 122, the threshhold value at which the transistor 128 will conduct can be adjusted.

In operation, referring to FIG. 4, the current limiter 62 has its first output lead 130 connected to the lead 136 while the other output lead 132 is connected to a common.

In operation of the current limiter 62, when the current transformer input from 64 indicates a high current flow in the primary 22, the output from th soft start circuit 58 is bled by the current limiter so that the gate drive 60 does not trigger the SCR gate 36, or the current limiter 62 retards the conduction angle and reduces the power output. The current limiter referred to in this invention works with proportional action, and does not necessarily shut off the SCRs 142 and 144, all the way.

Referring to the gate drive 60, the particular components which comprise the drive will not be discussed herein. Rather, reference will be made to a patent which discloses a similar type of gate drive. Reference is made to my previous U.S. Pat. No. 3,304,438 which is entitled Phase Shift Gate Drive Circuit. Referring to FIG. 4 of the present invention, the input power lead 147 is similarly connected to points 10, 53, and 54 in FIG. 8 of my previous patent. The input 136 of gate drive 60 is connected in a manner shown by reference numeral in FIG. 7 of my previous patent. The output leads 138 and 140 from the gate drive 60 are connected in a manner similar to 15 and 15' in FIG. 7 of my previous patent.

FIG. 9 illustrates the output signal 146 from the gate drive 60, relative to the AC line signal 150. The leading edge of the gate drive output signal is indicated by reference numeral 148. Depending upon the phase of the leading edge 148 more or less of the AC line signal 150 will be incorporated under a common area as shown shaded in FIG. 9. Thus, the gate drive 60 controls the power to the SCR gate by virtue of varying the phase relation. FIG. 10 illustrates an alternate method of detecting a spark. The discussed circuitry detected a spark by employing a resistor 38 (FIG. 1) which developed a signal that was transmitted to the logic 46. Although this approach operates satisfactorily, it would be more desirable to detect spark pulse voltage signals, such as 78, in the primary circuit of FIG. 1 rather than the high voltage secondary circuit discussed. Accordingly, the circuitry of FIG. 10 offers an extra advantage of isolation between the high voltage secondary circuitry and the logic circuitry. When using this alternate approach, the resistor 38 of FIG. 1 is removed so that a short circuit exists between point 44 and ground (42). The remainder of the circuitry discussed is identical.

However. in order to detect a spark, a second current transformer 152 is connected via leads 154 to the primary 156 ofa step-up transformer. The secondary 158 imposes a voltage across the parallel connected resistor 160. A diode bridge 162 rectifies the signal appearing across resistor 160. A rectified signal is tapped off at bridge points 166 and 168. These points are connected across the potentiometer 164. The lower terminal of the resistor is connected to ground as indicated at 170. The tap-off wiper 167 of the potentiometer 164 is connected to the logic 46 along with a lead from point 170. In effect, these output leads present a spark signal 78 in the same way that comparable leads supplied such a signal from across resistor 38 in the previous embodiment. In FIG. 10, the spark pluse voltage signal 78 occurs due to the transformer coupling of the secondaryprimary windings.

It should be understood that the invention is not limited to the exact details of construction shown and described herein for obvious modifications will occur to persons skilled in the art.

What is claimed is:

1. In a voltage control system for electrostatic precipitators comprising:

power input means for providing power to the system from power lines; a transformer; a primary of the transformer connected to the input means for developing an AC signal thereacross;

solid state gating means connected between the input means and the transformer primary for controlling power delivery to the primary;

first current sensing means coupled to the primary for developing a signal corresponding to current flow through the primary;

first switching means connected at the input thereof to the output of the sensing means for switching to a conductive state when a preselected threshold of the sensing means signal is exceeded indicating excessive current in the primary;

gate drive means connected at inputs thereof to the power input means and said first switching means, the output of the drive means connected to the gate means for triggering the gating means into conduction when said first switching means is in a nonconductive state;

the secondary of the transformer transforming the AC signal to a rectifier for rectification of the AC signal;

means connecting the rectifier to the precipitators for developing electrostatic charges thereon which attract particulatev matter passing across the precipitators; and

resistor means connected to said rectifier for developing voltage pulses thereacross when arcing occurs between said precipitators;

logic means connected to receive said voltage pulses from said resistor means and for producing first and second control voltages in response thereto; and

soft start analog circuit means including second switching means responsive to said first and second control voltages for momentarily disabling said gate drive means and subsequently causing gradual increased power delivery from the power lines to the gating means.

2. The subject matter as defined in claim 1 wherein the gating means comprises at least one pair of reverse positioned parallel connected SCRs, the anode'and cathode of each SCR conducting line current therethrough in response to trigger signals applied to gate terminals of the SCRs by the drive means.

3. The subject matter set forth in claim 1 together with an inductor connected in series with the transformer primary for inhibiting momentary increases in primary current.

4. A voltage control system for electrostatic precipitators comprising:

power input means for providing power to the system from power lines;

a transformer;

the primary of the transformer connected to the input means for developing an AC signal thereacross;

an inductor connected in series with the transformer primary for inhibiting momentary increases in primary current;

solid state gating means connected between the input means and the transformer primary for controlling power delivery to the primary, the gating means including a pair of parallel connected, reverse positioned SCRs, the anode and cathode of each SCR conducting line current therethrough'in response to trigger signals applied to the gate terminals of the SCRs;

first current sensing means coupled to the primary for developing a signal corresponding to current flow through the primary;

switching means including a transformer/rectifier set connected at the input thereof to the output of said first current sensing means, said switching means including means connected to the output of said transformer/rectifier set for establishing athreshold reference voltage and a first switching device responsive to the output of said establishing means for switching to a conductive state when a preselected threshold of the sensing means signal is exceeded, indicating excessive current in the primary;

gate drive means connected at inputs thereof to the power input means and the switching means, the output of the drive means connected to the gate terminals of the SCRs for generating the trigger signals when the switching means is in a nonconductive state;

the secondary of the transformer transforming the AC signal to a rectifier for rectification of the AC signal;

means connecting the rectifier to the precipitators for developing electrostatic charges thereon which attract particulate matter passing across the precipitators;

secondcurrent sensing means coupled to the primary for developing a signal corresponding to current flow through the primary;

circuit means connected to the output of the second sensing means for generating a pulse voltage signal from the second sensing means output in response to arcing at the precipitator; meansconnecting the circuit means to logic means for detecting the occurrence of the pulse voltage signal;

and soft start means connected between the logic means and the gate drive means, the soft start means being responsive when the logic means detects the pulse voltage for momentarily disabling the gate drive means and subsequently causing gradual increased power delivery from the power lines to the gating means;

the logic means comprising first and second circuit paths;

the first path including a one-shot for generating disabling pulses of predetermined duration in response to said pulse voltage signal;

a second switching device connected at its input to the one-shot output, the second switching device changing state for the duration of each disabling pulse, and means connecting the output of the switching means to the soft start means which momentarily disables the gate drive means in response to the change of state of the second switching means;

the second current path including a second one-shot having its input connected to the input of the first one-shot;

the second one-shot generating a squarewave pulse train, the pulses of the train being of constant width and amplitude but having a repetition rate equal to that of the pulses from the pulse voltage signal;

the output from the second one-shot connected to an operational amplifier for integrating the pulse train and forming a DC level therefrom, the DC level being an input to the soft start means for achieving the gradual increase of power delivered from the power lines to the gating means after the momentary disabling of the gate drive means.

5. The subject matter as described in claim 4 wherein the soft start means comprises:

a transformer having a primary and secondary; means connecting the primary to the power input means for impressing an AC signal thereacross;

means connecting the secondary to a rectifier for rec- I fecting rapid discharge thereof in the event of power interruption; and

a buffer transistor connected at its base to the charging circuit for disabling the gate drive means via the transistor emitter, in response to power interruption, the gate drive means subsequently achieving gradual increase of power delivered from the power lines to the gating means after disabling of the gate drive means;

the collector of the transistor connected to the output of the operational amplifier for conducting the DC level therefrom, through the transistor to the gate drive means;

the base of the buffer transistor being connected to the switching device of the logic means for causing momentary disabling of the gate drive means in response to arcing at the precipitators, the charging circuit achieving gradual increase of power delivformer.

7. The subject matter of claim 6 wherein the first and second switching devices are transistors. 

1. In a voltage control system for electrostatic precipitators comprising: power input means for providiNg power to the system from power lines; a transformer; a primary of the transformer connected to the input means for developing an AC signal thereacross; solid state gating means connected between the input means and the transformer primary for controlling power delivery to the primary; first current sensing means coupled to the primary for developing a signal corresponding to current flow through the primary; first switching means connected at the input thereof to the output of the sensing means for switching to a conductive state when a preselected threshold of the sensing means signal is exceeded indicating excessive current in the primary; gate drive means connected at inputs thereof to the power input means and said first switching means, the output of the drive means connected to the gate means for triggering the gating means into conduction when said first switching means is in a non-conductive state; the secondary of the transformer transforming the AC signal to a rectifier for rectification of the AC signal; means connecting the rectifier to the precipitators for developing electrostatic charges thereon which attract particulate matter passing across the precipitators; and resistor means connected to said rectifier for developing voltage pulses thereacross when arcing occurs between said precipitators; logic means connected to receive said voltage pulses from said resistor means and for producing first and second control voltages in response thereto; and soft start analog circuit means including second switching means responsive to said first and second control voltages for momentarily disabling said gate drive means and subsequently causing gradual increased power delivery from the power lines to the gating means.
 2. The subject matter as defined in claim 1 wherein the gating means comprises at least one pair of reverse positioned parallel connected SCRs, the anode and cathode of each SCR conducting line current therethrough in response to trigger signals applied to gate terminals of the SCRs by the drive means.
 3. The subject matter set forth in claim 1 together with an inductor connected in series with the transformer primary for inhibiting momentary increases in primary current.
 4. A voltage control system for electrostatic precipitators comprising: power input means for providing power to the system from power lines; a transformer; the primary of the transformer connected to the input means for developing an AC signal thereacross; an inductor connected in series with the transformer primary for inhibiting momentary increases in primary current; solid state gating means connected between the input means and the transformer primary for controlling power delivery to the primary, the gating means including a pair of parallel connected, reverse positioned SCRs, the anode and cathode of each SCR conducting line current therethrough in response to trigger signals applied to the gate terminals of the SCRs; first current sensing means coupled to the primary for developing a signal corresponding to current flow through the primary; switching means including a transformer/rectifier set connected at the input thereof to the output of said first current sensing means, said switching means including means connected to the output of said transformer/rectifier set for establishing a threshold reference voltage and a first switching device responsive to the output of said establishing means for switching to a conductive state when a preselected threshold of the sensing means signal is exceeded, indicating excessive current in the primary; gate drive means connected at inputs thereof to the power input means and the switching means, the output of the drive means connected to the gate terminals of the SCRs for generating the trigger signals when the switching means is in a non-conductive state; the secondary of the transformer transforming the AC sIgnal to a rectifier for rectification of the AC signal; means connecting the rectifier to the precipitators for developing electrostatic charges thereon which attract particulate matter passing across the precipitators; second current sensing means coupled to the primary for developing a signal corresponding to current flow through the primary; circuit means connected to the output of the second sensing means for generating a pulse voltage signal from the second sensing means output in response to arcing at the precipitator; means connecting the circuit means to logic means for detecting the occurrence of the pulse voltage signal; and soft start means connected between the logic means and the gate drive means, the soft start means being responsive when the logic means detects the pulse voltage for momentarily disabling the gate drive means and subsequently causing gradual increased power delivery from the power lines to the gating means; the logic means comprising first and second circuit paths; the first path including a one-shot for generating disabling pulses of predetermined duration in response to said pulse voltage signal; a second switching device connected at its input to the one-shot output, the second switching device changing state for the duration of each disabling pulse, and means connecting the output of the switching means to the soft start means which momentarily disables the gate drive means in response to the change of state of the second switching means; the second current path including a second one-shot having its input connected to the input of the first one-shot; the second one-shot generating a squarewave pulse train, the pulses of the train being of constant width and amplitude but having a repetition rate equal to that of the pulses from the pulse voltage signal; the output from the second one-shot connected to an operational amplifier for integrating the pulse train and forming a DC level therefrom, the DC level being an input to the soft start means for achieving the gradual increase of power delivered from the power lines to the gating means after the momentary disabling of the gate drive means.
 5. The subject matter as described in claim 4 wherein the soft start means comprises: a transformer having a primary and secondary; means connecting the primary to the power input means for impressing an AC signal thereacross; means connecting the secondary to a rectifier for rectifying the AC signal; means connected to the output of the rectifier for filtering the rectified signal; a charging circuit connected to the output of the filtering means for charging at a predetermined relatively slow rate when power is restored after an initial power interruption; diode means connected to the charging circuit for effecting rapid discharge thereof in the event of power interruption; and a buffer transistor connected at its base to the charging circuit for disabling the gate drive means via the transistor emitter, in response to power interruption, the gate drive means subsequently achieving gradual increase of power delivered from the power lines to the gating means after disabling of the gate drive means; the collector of the transistor connected to the output of the operational amplifier for conducting the DC level therefrom, through the transistor to the gate drive means; the base of the buffer transistor being connected to the switching device of the logic means for causing momentary disabling of the gate drive means in response to arcing at the precipitators, the charging circuit achieving gradual increase of power delivered from the power lines to the gating means after the momentary disabling.
 6. The subject matter defined in claim 5 wherein the current sensing means comprises a current transformer.
 7. The subject matter of claim 6 wherein the first and second switching devices are transistors. 